1. Field of the Invention
The present invention relates to a flow valve for controlling a the flow of fluid and, in particular, to a flow valve suitable for controlling flow of molten metal.
2. The Prior Art
Conventional valves, for controlling the flow of molten metal include plug valves which comprise a plug positioned above a hole, the plug being raised or lowered to unblock or block the hole as desired, and taper plugs, which generally comprise a conical plug retained within a conically tapering bore provided through a pipe, the taper plug having a passage drilled through it, such that by rotation of the taper plug about its conical axis within the bore the passage can be aligned to communicate or to block the flow of metal through the pipe.
It will be appreciated that while the simplicity of construction of such valves is an advantage when dealing with hot, and possibly corrosive, fluids such as molten metal, they allow for only coarse adjustment. In particular, they are inadequate when it is desired to maintain a very accurate rate of fluid flow or to dispense only a small, but accurate, amount of fluid. There is thus a need for a flow valve capable of dispensing fluid such as molten metal in an easily controllable and accurate manner. One example of an application where such control is needed is in the manufacture of melt-out cores for use in plastics moulding where accurate core size and quality of surface finish of the core are important features. Melt-out cores are usually made of readily fusible metals such as tin, lead and their alloys. The plastic is moulded around the core, which is then melted to leave a hollow plastic moulding.
Many different, and more complicated, designs of flow valve are known for controlling the flow of fluid such as water, aqueous solutions, oils, petroleum products and gases for use in either commercial or domestic applications. One type of valve which has been used comprises two valve members, one containing spaced inlet and outlet fluid passages and the other an intermediate passage, mounted such that relative rotation of the members can be effected to locate the intermediate passage either fully or partially in or out of communication with the inlet and outlet passages. Thus, by relative rotation of the members, the flow of fluid through the valve via inlet, intermediate and outlet passages can be controlled.
Valves of this general kind are known, for example, from GB 2064727, which describes and illustrates in FIG. 1 a stop or mixing valve; GB 1466904, which relates to mixing valves for liquids or gases and illustrates in FIG. 9 a mixing valve having three inflow and outflow passages in a lower valve chamber with an upper valve disc containing transfer channels pressed against the lower valve chamber by a compression spring stop; GB 1363835, which describes a valve for use in domestic pumping, for example a faucet; and GB 962936, which describes such a valve for use in gas chromatography.
Hitherto, such valves have not been used for fluids such as molten metal. One reason for this is that such fluids present additional problems. One major problem is the production of oxides. In molten metals maintained at an elevated temperature metal oxides are frequently produced and these oxides tend to collect as a precipitate together with other dross material at joins of moving parts present in a valve. If the valve is not regularly cleaned, these precipitates will cause it to become less efficient to operate and eventually to seize up. Flow valves of the kind traditionally used for molten metal also suffer from oxide build up, but their construction is sufficiently simple to allow for easy cleaning. In the case of a more complex and closely engineered valve, the build up of oxides can be far more serious.
The present invention provides a valve which is capable of accurate flow control of molten metal and which does not need frequent dismantling to remove oxide residues.